The present invention relates to roller conveyors, and particularly to conveyors of the type which include an endless belt of free rollers interconnecting conveyor chains which are driven along a conveyor path. The invention has particular application to means for effecting rotation of selected ones of the free rollers.
In a roller conveyor it is frequently desirable to effect selective rotation of certain ones of the rollers for various reasons. For example, the rollers along a certain section of the conveyor path may be rotated about their axes in a forward direction so that conveyed articles passing over that section will be accelerated, for the purpose of accumulating spaced-apart articles. Alternatively, rollers in a section of the conveyor path may be rotated in the reverse direction to retard conveyed articles as they pass over that section, such as for the purpose of introducing spaces between conveyed articles.
Typically, this selective rotation of the free rollers is effected by traction means, such as a traction shoe, which is movable into frictional engagement with the surface of the roller. As the conveyor belt moves past the traction shoe, the engagement of the rollers with the traction shoe effects a rotation of the rollers about their axes, the direction of rotation being dependent upon whether the traction shoe is disposed above or beneath the rollers. Such an arrangement is disclosed, for example, in U.S. Pat. No. 4,142,624.
This prior arrangement for effecting roller rotation has a number of drawbacks. First of all, the rollers and the traction shoes must be manufactured to careful tolerances, as must the drive means for the traction shoes, to insure that the traction shoes will be brought into sufficient contact with the rollers to effect rotation thereof, but will not bear against the rollers with excessive force, which can cause undue wear on the rollers and the traction shoes. If the rollers are untrue, or if the axes of adjacent rollers are not coplanar, then the rollers will not be evenly engaged by the traction shoes and skipping and uneven rotation results. Furthermore, frequently the rollers have oil on them which causes the traction shoes to slide on the rollers, thereby adversely affecting proper rotation and tending to increase wear on the rollers and the shoes. The traction shoes must be moved into engagement with the rollers with substantial force in order to allow for these tolerance considerations and, therefore, both the friction shoes and the rollers tend to wear out rather rapidly.
Another drawback of the prior arrangement is that it lacks flexibility. For example, typically, the traction shoe is an elongated member which is adapted to engage a plurality of adjacent rollers. In this arrangement, it is very difficult to selectively omit rollers from rotation. Thus, for example, if it were desired to effect rotation of only alternate rollers in a region of the conveyor path, it would be necessary to use a large number of small shoes, each adapted to engage only a single roller at a time, a complicated and expensive option, or it would be necessary to change the lengths of alternate rollers so that the shorter ones would not engage the traction shoe.
Similarly, in prior arrangements, the speed of rotation of the rollers by the traction shoes is dependent upon the speed of movement of the conveyor belt and cannot be altered unless the belt speed is altered.